Such servos conventionally include a casing including a cylinder and a cover having an axis of symmetry, the inside of the casing being divided in leaktight fashion by a movable wall structure into a front chamber permanently connected to a source of partial vacuum, and a rear chamber connected selectively to the front chamber or to a source of pressure by a three-way valve located in a rear tubular part of the movable wall structure sliding in a leaktight fashion in the cover and actuated by a control rod.
Very many documents relate to this type of pneumatic servo. In all these known embodiments, the casing of the servo is typically formed of a front half-shell, also termed cylinder, and of a rear half-shell, also termed cover, the cylinder and cover being secured to one another by crimping, trapping between them an outer peripheral bead of the unrolling diaphragm providing sealing between the front and rear chambers.
The cover is conventionally intended to be fixed to the bulkhead of the vehicle, on the engine compartment side, and includes an axial cylindrical extension, also termed shaft, pointing toward the rear and penetrating into the passenger compartment of the vehicle. This shaft receives, with leaktight sliding, the rear tubular part of the movable wall structure containing the three-way valve.
The cylinder for its part includes a plane face around the axis of symmetry of the servo, which face is intended to receive a fastening flange of a master cylinder, actuated by a pushrod secured to the movable wall structure of the servo, when the latter is actuated.
It follows that the cylinder and the cover, together constituting the casing of the servo, form two different components. This implies that the time of design, manufacture, storage and management costs are multiplied by two. In particular, the cover including the shaft described above constitutes a component which is quite tricky and expensive to manufacture owing to its complex shape, rejects on the production or assembly line not being negligible.